This invention relates to an apparatus and method for controlling an intermittent supply operation performed by a supply device used in the transfer of a product in the form of solid particles. The supply device, forming part of the transfer system, is of the kind located upstream of a portion of the transfer system where the product is temporarily pooled at some point along the transfer path.
In a product transfer arrangement often adopted in automated production lines, a given product is transferred between points while the product is subjected to various processes en route, with processing ending at the terminus of the transfer path. An excellent example of such an arrangement is an automated weighing and packaging line for feeding a product to be weighed, which consists of a mass of solid particles, to a weighing apparatus where the product is weighed, separating from the majority of the product a batch thereof having a total weight conforming to a target weight, and packaging the batch. Such an arrangement can take on a very sophisticated form, as by combining a plurality of the weighing apparatus.
In a product transfer path of the kind described, each process (such as weighing) performed therealong requires a certain period of time. Accordingly, the product flow is inevitably of an intermittent nature, and the instantaneous flow rate when the product is discharged from the system is much larger in comparison with the average amount of flow. In order to supply a processing apparatus, such as the weighing apparatus, with the product without interruption, installation of a supply device which operates at the discharge flow rate of the processing apparatus, namely a supply device having a supply capability equivalent to that of the instantaneous flow rate mentioned above, is undesirable. The reason is that a supply device having such a capability would be large in size, high in cost, wasteful of energy and prone to breakdown due to shock caused by frequent starting and stopping.
In order to supply an intermittently operating processing apparatus of the kind described above with the product at a flow rate which is as close to an average flow rate as possible, conventional practice is to cope with the changing flow rate of the processing apparatus by providing a pooling unit, at some point along the supply path, for temporarily pooling an appropriate amount of the product. The pooling unit is provided with a sensor for sensing the amount of product pooled. This is achieved by adapting the sensor to sense the level to which the product has risen within the pooling unit, and to produce a signal indicative of the sensed level. In response to this signal, the supply device, which is located upstream of the pooling unit, is controlled so as to operate in intermittent fashion. With this arrangement, however, the supply device is operated for intervals which are comparatively long.
Where the processing apparatus is a weighing apparatus, the pooling unit ordinarily is provided at the inlet to the weighing apparatus. The pooling unit is, in many cases, referred to as a hopper which comprises a vessel in the shape of an inverted cone having large and small openings at the upper and lower ends thereof, respectively. Depending upon the design of the weighing apparatus, however, a product may be supplied by dropping from the central portion of the apparatus without use of a vessel, with the product being accumulated in the form of pile formed when the product is discharged from the weighing apparatus. In either case, the quantity of the product pooled is sensed based upon the height of the accumulated heap.
The supply device used to supply the weighing apparatus usually is an electromagnetic feeder, conveyor or the like. In response to generation of a supply request signal, a trough is vibrated by an electromagnet in a case where the supply device is an electromagnetic feeder, or a conveyor is driven if the supply device is of the conveyor type. In either case, the product is delivered to the pooling unit where, when the product accumulates to a prescribed amount, a switch is actuated to terminate the product feed to the pooling unit. The purpose of this system is to automate the supply operation.
The sensor used to sense the amount of the product which has accumulated within the pooling unit generally comprises a light-emitting element and a light-receiving element.
FIG. 1 is an example of a prior art arrangement, in which the supply device and pooling unit are a conveyor 11 and a hopper 13, respectively. By driving the conveyor 11 in the direction of the arrow, a product 12 carried by the conveyor is fed into the hopper 13. A light-emitting element 14 and a light-receiving element 15 are disposed on either side of the hopper 13 at a prescribed height from the bottom of the hopper, which has transparent windows provided in the walls thereof at positions corresponding to the elements 14 and 15. The arrangement is such that a light beam emitted from the light-emitting element 14 impinges upon the light-receiving element 15 by passing through the transparent windows. The amount (i.e., level) of the product within the hopper 13 is sensed when the product piles up high enough to intercept the light beam. The product is discharged from an opening (not shown) located at the lower end of the hopper in an amount commensurate with the amount to be processed by the weighing apparatus.
Since the sensor composed of the elements 14 and 15 generally has excellent sensitivity, the output DOS of the sensor frequently makes a transition between high and low levels, as shown by interval A in FIG. 2, when the product passes through the light beam in the course of dropping into the hopper, and when the product has risen to a level close to the level of the sensor. (The interval starting at B in FIG. 2 shows that the state of the output signal DOS is stable when the product has reached a level high enough to completely interrupt the light beam.) As a result of the behavior of the signal DOS in interval A, a switch, which controls the starting and stopping of the product feed in response to the signal DOS, is opened and closed very often. If this chattering action of the switch also causes the supply device to turn on and off at the same frequency, supply of the product will not take place in normal fashion.
A conventional method of eliminating the influence of the frequent transitions made by the sensor output DOS is to combine the foregoing arrangement with a timer or other element which is not a sensor. Specifically, when the above-described sensor produces the output DOS in response to interception of the light beam, a timer (a software timer implemented by a CPU), which is set to a time T, starts operating. If the level of the signal DOS changes owing to reception of the light beam before the expiration of the time T, the timer operation is cancelled and the timer is reset to T. The foregoing cycle is repeated by any subsequent interception of the light beam.
The supply device stops operating depending upon the sensor output DOS and the output of the timer following expiration of the time T. That is, operation of the supply device is terminated upon lapse of a certain time T after the sensor output DOS leaves the unstable region (interval A) and enters the stable region (interval starting at B).
With the above method, only the sensor and timer develop chatter in the unstable region, whereas the supply device is stably driven.
In a case where the supply device starts to be driven in response to reception of the light beam in the sensor, a region of instability at the start of operation is avoided by a method similar to that just described. As a result, the supply device may supply the product in a normal manner without being affected by chattering in the region of instability (interval A). The amount of the product supplied during the time T following stabilization of the sensor output DOS is that needed for the product to pile up above the level of the sensor. This amount is substantially equivalent to that needed to charge the hopper.
By using the sensor and timer in combination, the single sensor enables upper and lower limits on the amount of product pooled to be set, and makes it possible to control the starting and stopping of the supply device. However, with the conventional method described above, supply is terminated upon passage of the time T, measured from the instant the stable region B is reached, irrespective of the amount of product supplied per time unit (the supply flow rate, represented by Q). Consequently, the amount of charge, namely the total quantity supplied following arrival at the prescribed level, will vary considerably. This represents a major disadvantage encountered with the conventional method. By way of example, if the flow rate is Q1, indicating a small amount of feed per unit time, as shown in FIG. 3, the total amount of product supplied following arrival at the prescribed level will be Q1.multidot.T. If the flow rate is Q2 (&gt;Q1), indicating a large amount of feed per unit time, as shown in FIG. 4, then the total amount of product supplied will be Q2.multidot.T. Therefore, the total amounts supplied are unequal, i.e., Q1.multidot.T.noteq.Q2.multidot.T.
In a combinatorial weighing apparatus, the combinatorial weighing precision declines with too large a variation in the amount of product supplied to each of the weighing machines (weighing hoppers). Specifically, the transfer of the product from the pooling unit to the weighing machines is performed by electromagnetic feeders. A difference in the amount of product supplied is brought about by the fact that the operating time of the feeders is preadjusted by timer settings, and by the fact that a product in the form of solid particles exhibiting flowability has a nature somewhat analagous to that of a fluid. In other words, the foregoing results in a difference in level (potential head) within the pooling unit, which in turn brings about a difference in the product layer thickness in the transfer path downstream. Even if the feeders are operated for the same length of time, therefore, the amount of product supplied will vary.
On the other hand, if those weighing machines left empty by discharging the product are resupplied by an amount of the product which differs greatly (on the high or low side) from a target weight value, irrespective of the fact that the weighing machines discharge the product at the fixed target weight as the result of combinatorial weighing, obviously the probability of finding combinations suitable for subsequent discharge cycles will diminish. With the conventional method described above, the amount supplied per unit time (i.e., Q) varies depending upon the production process, resulting in a change in the amount of the product supplied to the weighing machines. This is an obstacle in obtaining a highly precise combinatorial weighing operation.